US20170009036A1 - Structured Porous Metamaterial - Google Patents

Structured Porous Metamaterial Download PDF

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US20170009036A1
US20170009036A1 US15/113,373 US201515113373A US2017009036A1 US 20170009036 A1 US20170009036 A1 US 20170009036A1 US 201515113373 A US201515113373 A US 201515113373A US 2017009036 A1 US2017009036 A1 US 2017009036A1
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base unit
void
metamaterial
base
voids
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Yi Min Xie
Jianhu Shen
Shiwei Zhou
Xiaodong Huang
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RMIT University
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RMIT University
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    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J9/00—Working-up of macromolecular substances to porous or cellular articles or materials; After-treatment thereof
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C44/00—Shaping by internal pressure generated in the material, e.g. swelling or foaming ; Producing porous or cellular expanded plastics articles
    • B29C44/34—Auxiliary operations
    • B29C44/35—Component parts; Details or accessories
    • B29C44/355—Characteristics of the foam, e.g. having particular surface properties or structure
    • B29C44/357—Auxetic foams, i.e. material with negative Poisson ratio; anti rubber; dilatational; re-entrant
    • B29C67/0051—
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y10/00—Processes of additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y70/00—Materials specially adapted for additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B33—ADDITIVE MANUFACTURING TECHNOLOGY
    • B33Y—ADDITIVE MANUFACTURING, i.e. MANUFACTURING OF THREE-DIMENSIONAL [3-D] OBJECTS BY ADDITIVE DEPOSITION, ADDITIVE AGGLOMERATION OR ADDITIVE LAYERING, e.g. BY 3-D PRINTING, STEREOLITHOGRAPHY OR SELECTIVE LASER SINTERING
    • B33Y80/00—Products made by additive manufacturing
    • G—PHYSICS
    • G01—MEASURING; TESTING
    • G01N—INVESTIGATING OR ANALYSING MATERIALS BY DETERMINING THEIR CHEMICAL OR PHYSICAL PROPERTIES
    • G01N3/00—Investigating strength properties of solid materials by application of mechanical stress
    • G01N3/08—Investigating strength properties of solid materials by application of mechanical stress by applying steady tensile or compressive forces
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29C—SHAPING OR JOINING OF PLASTICS; SHAPING OF MATERIAL IN A PLASTIC STATE, NOT OTHERWISE PROVIDED FOR; AFTER-TREATMENT OF THE SHAPED PRODUCTS, e.g. REPAIRING
    • B29C64/00—Additive manufacturing, i.e. manufacturing of three-dimensional [3D] objects by additive deposition, additive agglomeration or additive layering, e.g. by 3D printing, stereolithography or selective laser sintering
    • B29C64/10—Processes of additive manufacturing
    • B—PERFORMING OPERATIONS; TRANSPORTING
    • B29—WORKING OF PLASTICS; WORKING OF SUBSTANCES IN A PLASTIC STATE IN GENERAL
    • B29K—INDEXING SCHEME ASSOCIATED WITH SUBCLASSES B29B, B29C OR B29D, RELATING TO MOULDING MATERIALS OR TO MATERIALS FOR MOULDS, REINFORCEMENTS, FILLERS OR PREFORMED PARTS, e.g. INSERTS
    • B29K2083/00—Use of polymers having silicon, with or without sulfur, nitrogen, oxygen, or carbon only, in the main chain, as moulding material
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2205/00—Foams characterised by their properties
    • C08J2205/04—Foams characterised by their properties characterised by the foam pores
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2300/00—Characterised by the use of unspecified polymers
    • C08J2300/26—Elastomers
    • C—CHEMISTRY; METALLURGY
    • C08—ORGANIC MACROMOLECULAR COMPOUNDS; THEIR PREPARATION OR CHEMICAL WORKING-UP; COMPOSITIONS BASED THEREON
    • C08J—WORKING-UP; GENERAL PROCESSES OF COMPOUNDING; AFTER-TREATMENT NOT COVERED BY SUBCLASSES C08B, C08C, C08F, C08G or C08H
    • C08J2383/00—Characterised by the use of macromolecular compounds obtained by reactions forming in the main chain of the macromolecule a linkage containing silicon with or without sulfur, nitrogen, oxygen, or carbon only; Derivatives of such polymers
    • C08J2383/04—Polysiloxanes

Definitions

  • the present invention generally relates to a three dimensional (3D) structured porous metamaterials with specific deformation pattern under applied loading, and more particularly a 3D structured porous metamaterials having a negative or zero Poisson's ratio and/or zero or negative compressibility (NC).
  • 3D structured porous metamaterials having a negative or zero Poisson's ratio and/or zero or negative compressibility (NC).
  • a material's Poisson's ratio is defined as the negative of the ratio of that materials lateral strain to its axial strain under uniaxial tension or compression. Most materials have a positive Poisson's ratio and therefore which expand laterally under compression and contract in the transverse direction under axial tension. Auxetic materials are materials with negative Poisson's ratio (NPR). The materials contract laterally under compression and expand in the transverse direction under axial tension.
  • NPR negative Poisson's ratio
  • Compressibility is a measure of the relative volume change of a solid or fluid as a response to a pressure change. Usually a material contracts in all directions when the pressure increases. However there are some exceptional materials which expand under hydrostatic pressure in one or two directions. Such phenomena are known as negative linear compressibility (NLC) and negative area compressibility (NAC), respectively.
  • NLC negative linear compressibility
  • NAC negative area compressibility
  • Overvelde et al (Compaction Through Buckling in 2D Periodic, Soft and Porous Structures: Effect of Pore Shape. Advanced Materials. 2012; 24:2337-2342) teaches two dimensional soft cellular structures that comprise a solid matrix with a square array of holes. No three dimensional structures are investigated. The response of 2D porous structure to compression, including the Poisson's ratio of the material, are taught as being designed and tuned by changing the shape of the holes. Structures with a porosity ⁇ of between 0.4 and 0.5 were identified as providing suitable auxetic properties. Structures with smaller porosity were noted to facilitate macroscopic instability leading to structures characterised by limited compaction. Structures with higher levels of porosity where also noted as leading to structures characterised by very thin ligaments, making them fragile.
  • United States Patent Publication No. 20110059291 A1 teaches both two dimensional and three dimensional structured porous materials having a porous structure provides a range in Poisson's ratio ranging from a negative Poisson's ratio to a zero Poisson's ratio.
  • the geometry of the voids is suggested as being variable over a wide range of sizes and shapes.
  • the exemplar structures consist of a pattern of elliptical or elliptical-like voids in an elastomeric sheet.
  • the porous pattern of both two dimensional and three dimensional comprise a matrix of voids having a porosity ⁇ of less than 0.5.
  • the voids are located in the matrix as individual shapes within the base material, and are spaced apart in a regular pattern.
  • Babaee et al (3D soft metamaterials with negative Poisson's ratio. Advanced Materials. 2013; DOI: 10.1002/adma.201301986:1-6) teaches a new class of three-dimensional metamaterials with negative Poisson's ratio.
  • a library of auxetic building blocks is identified and procedures are defined to guide their selection and assembly.
  • the taught materials all comprise a three dimensional matrices of ball shaped building block units. Each ball building block includes shaped voids. The balls are stacked in a complex three dimensional array to form the metamaterial.
  • NPR negative Poisson's ratio
  • NLC negative linear compression
  • NAC negative area compression
  • ZLC zero linear compression
  • ZAC zero area compression
  • this new auxetic metamaterial has a different and/or simpler structure than the metamaterial taught in Babaee et al.
  • the present invention provides in a first aspect a structured porous metamaterial comprising a three-dimensional matrix of at least one repeating base unit, the matrix formed from an array of at least eight base units, each base unit comprising a platonic solid including at least one shaped void, wherein the geometry of the at least one shaped void of each base unit is tailored to:
  • the present invention can therefore provide two broadly different properties through the inventive porous structure:
  • the present invention provides a structured porous metamaterial having a response under tension and compression having a Poisson's ratio of 0 to ⁇ 0.5.
  • This embodiment of the present invention comprises a simple building unit that provides a large and tuneable negative Poisson's ratio (NPR) strain range under both tension and compression.
  • NPR negative Poisson's ratio
  • the negative and/or zero Poisson's ratio behavior of this metamaterial is a result of the mechanics of the deformation of the voids and the mechanics of the deformation of the solid base material.
  • the porosity is preferably between 0.30 and 0.97. More preferably, the porosity is:
  • the present invention provides a structured porous metamaterial comprising a three-dimensional matrix of at least one repeating base unit, the matrix formed from an array of at least eight base units, each base unit comprising a platonic solid including at least one shaped void, wherein the geometry of the shaped void of each base unit is tailored to:
  • the size and geometry of the void needs to be configured to provide a porosity ⁇ of between 0.69 and 0.965 in the metamaterial with base unit comprising a cube with a spherical shaped void in order to provide the advantageous negative and/or zero Poisson's ratio behavior for the defined base unit.
  • the inventors have found that lower porosity values as taught as being essential in US20110059291 and Overvelde et al do not provide a three dimensional porous structure which displays tuneable negative and/or zero Poisson's ratio over a large compression strain, despite these characteristics being demonstrated as being displayed in the two and three dimensional structures.
  • the desired properties and deformation characteristic of those materials can only be reproduced in three-dimensional structure through significant modification of the porous structure and geometry of the base unit and constituent void.
  • the inventors consider that the negative Poisson's ratio of the metamaterial of the present invention is achieved through selection of the geometry and porosity of the material to create a desired alternating opening and closing deformation pattern of the voids and a specific configuration of the base unit which on compression allows spatial rotation and translation of part of the material of the base unit accompanied by the bending and stretching of other parts of the material of the base unit.
  • the present invention provides a structured porous metamaterial having a negative linear compression (NLC), negative area compression (NAC), zero linear compression (ZLC), or zero area compression (ZAC) behavior when under pressure.
  • NLC negative linear compression
  • NAC negative area compression
  • ZLC zero linear compression
  • ZAC zero area compression
  • the metamaterial comprise a simplified building unit that provides NLC, NAC, ZLC, ZAC behaviour under pressure.
  • these building units are derived from bi-directional evolutionary structural optimization (BESO).
  • the porosity is preferably between 0.30 and 0.97. More preferably, the porosity is between 0.3 and 0.95 for optimised shaped voids.
  • the present invention provides in a structured porous metamaterial comprising a three-dimensional matrix of at least one repeating base unit, the matrix formed from an array of at least eight base units, each base unit comprising a platonic solid including at least one optimised shaped void, wherein the geometry of the at least one shaped void of each base unit is tailored to:
  • the matrix structure of the metamaterial of the present invention is formed from repeating adjacent base units.
  • the metamaterial is formed from a three dimensional matrix formed from an array of at least eight base units, preferably arranged as a 2 ⁇ 2 ⁇ 2 matrix and preferably many more than eight base units arranged in a three dimensional matrix.
  • the shape of the base unit is a platonic solid which enables the base unit to be arranged in a matrix without any voids or gaps between adjacent units.
  • the base unit comprises at least one of a tetrahedron, cube, cuboid, parallelepiped, octahedral, dodecahedron, or icosahedron.
  • the base unit comprises a six sided shape, preferably a cube, cuboid, parallelepiped, and more preferably a cube, more preferably a cubic symmetric platonic solid.
  • Each base unit includes a geometric center.
  • the geometry of the void is centered about the geometric center of the base unit, and more preferably the geometric center of each void is centered about the geometric center of the base unit. This provides a regular spacing between the center of adjacent void shapes throughout the matrix.
  • the negative Poisson ratio of the metamaterial can be tuned by using different base shape for the void and buckling mode of the representative element.
  • a material formed from a base unit including a void having a spherical base shape has a different negative Poisson ratio to a material formed from a base unit including a void having an ovoid base shape.
  • a material formed from a base unit including a void having a spherical base shape or an ovoid base shape has a different negative Poisson ratio to a material formed from a base unit including a void having an ellipsoid shape.
  • the void or voids within each base unit can have any suitable shape and configuration.
  • the base shape of the void is preferably selected to provide desired tension and compression properties to the metamaterial.
  • the base geometric shape of the voids comprises a spherical shape or at least one regular non-spherical shape such as ovoid, ellipsoid (including rugby ball shaped), cubic, cuboid, parallelepiped, hyperboloid, conical, octahedron, or other regular 3D polygon shape.
  • the void comprises a spherical, ovoid, or ellipsoid, more preferably spherical, or ovoid, and yet more preferably spherical.
  • the void or voids can have a non-regular shape.
  • the void or voids can be formed from a combination of interconnected void shapes such as ovoid, ellipsoid (including rugby ball shaped), cubic, cuboid, parallelepiped, hyperboloid, conical, octahedron, or other regular 3D polygon shape.
  • the base geometric shape of the voids comprises an optimised shape, thus comprising an optimised shape void.
  • an optimised shaped void is a shaped void having a configuration and shape derived from optimization algorithms, preferably bi-directional evolutionary structural optimization (BESO), to provide the desired response properties.
  • BESO bi-directional evolutionary structural optimization
  • the void shape is therefore has an optimised shape to provide these responses.
  • optimised shaped voids typically have complex shapes and can comprise an amalgamation of a number of different regular shapes.
  • optimised shaped voids can comprise two or more separate void shapes within the base unit.
  • a base unit may include three separate void spaces, the void spaces being generally located at the sides and one void around the geometric center of the base unit.
  • the void is shaped to assist in providing the metamaterial with at least one of a negative linear compression (NLC), negative area compression (NAC), zero linear compression (ZLC), or zero area compression (ZAC) behavior when under pressure.
  • NLC negative linear compression
  • the porosity of the metamaterial and constituent base unit is an essential factor in the deformation characteristics of the metamaterial of the present invention.
  • the porosity of the base unit is typically configured to be between 0.3 and 0.97. In preferred embodiments, the porosity is between 0.4 and 0.90, and more preferably between 0.50 and 0.90. In some embodiments, the porosity is between 0.60 and 0.90. In some embodiments, the porosity is between 0.3 and 0.80. In some embodiments, the porosity is between 0.69 and 0.90. In some embodiments, the porosity is between 0.50 and 0.97. In some embodiments, the porosity is between 0.60 and 0.97.
  • the effective porosity varies with the shape of void in the building cell.
  • the void geometry of the base unit is preferably be tailored to provide a porosity of:
  • the porosity is preferably between 0.69 and 0.97. In those embodiments in which the metamaterial comprises a cubic base unit with an ellipsoid void, the porosity is preferably between 0.3 and 0.875. In those embodiments in which the metamaterial includes an optimised shaped void the porosity is between 0.3 and 0.97 for optimised shaped voids, preferably between 0.40 and 0.90, and more preferably between 0.50 and 0.90.
  • the base unit comprises a platonic solid.
  • optimised shaped voids the shaped void or voids in the base unit form spaces within that platonic solid which cut out or shape the solid material in the unit cell into the required form to provide the desired NLC, NAC, ZLC or ZAC property.
  • optimised shaped voids geometries are determined using optimization algorithms, for example bi-directional evolutionary structural optimization (BESO), to provide a unit cell structure with those properties.
  • BESO bi-directional evolutionary structural optimization
  • the base unit typically includes a width, height and length.
  • at least one dimension of the base geometric shape of the void is larger than at least one of the width, height or length of the base unit.
  • the void comprises a truncated form of a base geometric shape.
  • the base geometric shape of the void comprises a sphere and the base unit comprises a cube
  • the diameter of the sphere can be greater than the width, height and length of the cubic base unit.
  • the base geometric shape of the void comprises an ellipsoid and the base unit comprises a cube
  • selected diameters of the ellipsoid can be greater than the width, height and length of the cubic base unit.
  • the shape of the void will then be a truncated ellipsoid shape.
  • the void forms openings in the side of the base unit shape.
  • the void includes an opening in at least one, preferably two sides of the base unit.
  • the base geometric shape of the void comprises a sphere and the base unit is cubic
  • the base spherical geometric shape would form circular openings in each of the side walls of the cubic base unit.
  • the void includes an opening in at least two opposing sides of the base unit. In this way, the void space of a first base unit is interconnected to the void space of at least two adjacent base units.
  • the void includes at least one opening in each (all) sides of the base unit.
  • the configuration of the base unit, void geometry and pattern of the matrix formed from the base units can be tailored using a buckling mode obtained through Finite Element analysis, so that it provides a means to control the initial value of Poisson's ratio ranging from 0 to ⁇ 0.5.
  • the desired deformation state of the material comprises adjacent voids being alternatively open and closed throughout the matrix. It can be advantageous to pattern the voids into that deformation pattern in order to force the voids to take that configuration when the material is subject to tension or compression.
  • the base geometric shape of the void comprises shape having a greater central length than central height, the shape having a central length axis, the matrix of base units being arranged such that the central length axis of the void of each base unit is perpendicular to the central length axis of the void of each adjoining base unit.
  • the void shape comprises an ovoid or an ellipsoid, more preferably an ovoid.
  • the metamaterial can comprise a three-dimensional matrix of at least two different repeating base units, comprising a first base unit comprising a platonic solid including at least a first shaped void and a second base unit comprising a platonic solid including a second shaped void.
  • the first base unit and second base unit are preferably arranged in a pattern, preferably a regular pattern in the three-dimensional matrix.
  • the first shaped void has a different shape to the second shaped void.
  • the voids can have any suitable form.
  • the voids comprise an empty space framed by the material of the base unit.
  • the voids are composed of a compressible material, preferably having a high compressibility.
  • the voids include at least one fluid, preferably at least one liquid.
  • the geometry of the voids in the base unit is configured to allow the fluid flow through the voids in the matrix.
  • filling such voids with a fluid where the fluid acts as a dampening mechanism is preferred.
  • the base unit material can be any suitable base material.
  • the base unit material comprises a polymeric material.
  • Exemplary polymeric materials include at least one of an unfilled or filled vulcanized rubber, natural or synthetic rubber, crosslinked elastomer, thermoplastic vulcanizate, thermoplastic elastomer, block copolymer, segmented copolymer, crosslinked polymer thermoplastic polymer, filled or unfilled polymer or epoxy.
  • base unit material comprises metallic and ceramic and composite materials. Exemplary metals include aluminium, magnesium, titanium, iron and alloys thereof.
  • the base unit material comprises a biocompatible material, preferably a biocompatible polymeric material.
  • the structure and configuration of the metamaterial of the present invention can be determined using a number of methods.
  • the configuration of a structured porous metamaterial according to the present invention is determined using structural optimisation algorithms, such as a bi-directional evolutionary structural optimization (BESO) modelling techniques.
  • BESO bi-directional evolutionary structural optimization
  • a second aspect of the present invention provides a method of determining the configuration of a structured porous metamaterial comprising a three-dimensional matrix of at least one repeating base unit, comprising:
  • the configuration of the shaped voids within each base unit is derived from a bi-directional evolutionary structural optimization (BESO) model.
  • BESO evolutionary structural optimization
  • the step of simplifying the configuration of the at least one shaped void of each base unit is aimed at simplifying and/or optimizing the configuration of the base unit and resulting matrix for 3D printing construction.
  • This step therefore preferably comprises reconfiguring the topology of the shaped void or voids to have a more regular geometric shape.
  • This simplified configuration is typically more suitable for 3D printing construction.
  • this method is suitable for forming a structured porous metamaterial according to the first aspect of the present invention.
  • the method of this second aspect is particularly suitable for forming metamaterial of the second embodiment of the first aspect of the present invention comprising optimised shaped voids which provide a structured porous metamaterial having a negative linear compression (NLC), negative area compression (NAC), zero linear compression (ZLC), or zero area compression (ZAC) behavior when under pressure.
  • NLC negative linear compression
  • NAC negative area compression
  • ZLC zero linear compression
  • ZAC zero area compression
  • the metamaterial of the present invention has potential to be used as a mechanism for redistributing the base material of the metamaterial according to the external loads so as to support external loading more effectively.
  • Such a designed structural anisotropy can guide the loading into certain directions.
  • this type of metamaterial could be designed to create complex stress-strain paths to protect a certain internal volume.
  • the tunable Poisson's ratio and/or compressibility of the present invention are a result of determining the deformation characteristics of the metamaterial during buckling of the structure when a force, preferably a compression force or pressure is applied to the material. This can be determined using a standard buckling analysis of the material, in which the deformation mechanism is determined.
  • the deformation characteristics at buckling are termed the “buckling mode” of the base unit.
  • the buckling mode provides the structure of deformation of the material.
  • the structure of the base unit and more preferably of the void can then be modified to change (enhance or inhibit) the initial microstructure of the initial metamaterial and thus change/tune properties of the metamaterial such as the value of Poisson's ratio, effective strain range and/or compressibility for the desired NPR, NLC, NAC, ZLC, and/or ZAC behaviour of the material.
  • the present invention provides in a third aspect, a method of tuning the value of Poisson's ratio and effective strain range of a metamaterial according to the first aspect of the present invention.
  • the method includes the steps of:
  • the shape of the void of the base unit is altered to modify the configuration of the base unit.
  • FIG. 1A provides the geometric configurations for a comparative three dimensional structure porous material without negative Poisson's ratio showing (A) the base cell unit; (B) a block of the comparative material comprising an 8 ⁇ 8 ⁇ 8 matrix of the base unit; and (C) representative volume unit of the comparative material.
  • FIG. 1B provides the geometric configurations for a three dimensional structure porous metamaterial according to the first embodiment of the present invention showing (A) the base cell unit; (B) a block of the inventive metamaterial comprising an 8 ⁇ 8 ⁇ 8 matrix of the base unit; and (C) representative volume unit of the inventive metamaterial.
  • FIG. 2 provides photographs of samples of the metamaterial shown in FIG. 1B (A) with supporting material fabricated using 3D printing and (B) without supporting material fabricated using 3D printing.
  • FIG. 3A shows the deformation patterns and thus buckling model for materials with (A) comparative a face-centred cubic cell (volume fraction: 51.0%) and (B) a cubic building cell according to the present invention (volume fraction: 12.6%).
  • FIG. 3B provides a view of force-displacement response of inventive metamaterial in two different directions D 1 and D 2 .
  • FIG. 3C provides a comparison of nominal stress-strain curve of comparative structure porous material with face-centred cubic cell along three different loading directions.
  • FIG. 4 provides a comparison of deformation pattern of inventive metamaterial (volume fraction: 12.6%, load direction: D 2 ( FIG. 3 ), strain rate: 10 ⁇ 3 s ⁇ 1 ) between (A) experiment and (B) finite element model.
  • FIG. 5 provides a comparison of nominal stress-strain curve of inventive metamaterial between experiment and finite element model for spherical voids and slightly ovoid shaped voids shaped (spherical with imperfection).
  • FIG. 6 provides a comparison of deformation pattern of an embodiments of the inventive metamaterial including slightly ovoid shaped voids (volume fraction: 12.6%, strain rate: 10 ⁇ 3 s ⁇ 1 ) between (A) experiment and (B) finite element model.
  • FIG. 7A provides the geometric configurations for a three dimensional structure porous metamaterial with tetrahedron in cube building cell, showing (A) the base cell unit; (B) a block of the inventive metamaterial comprising an 8 ⁇ 8 ⁇ 8 matrix of the base unit; and (C) an isometric view of the representative volume unit of the inventive metamaterial.
  • FIG. 7B provides the geometric configurations for a three dimensional structure porous metamaterial with ellipsoid in cube building cell, showing (A) the base cell unit; (B) a block of the inventive metamaterial comprising an 8 ⁇ 8 ⁇ 8 matrix of the base unit; and (C) an isometric view of the representative volume unit of the inventive metamaterial.
  • FIG. 8A provides the deformation pattern for the metamaterial shown in FIG. 7A under load, showing (A) deformation pattern for bulk material (8 ⁇ 8 ⁇ 8) in xz plane; (B) deformation pattern for bulk material (8 ⁇ 8 ⁇ 8) in yz plane; (C) deformation pattern for the representative volume unit (2 ⁇ 2 ⁇ 2) in xz plane; and (D) an isometric view of the deformation pattern of the representative volume unit (2 ⁇ 2 ⁇ 2).
  • FIG. 8B provides the deformation pattern for the metamaterial shown in FIG. 7B under load, showing (A) deformation pattern for bulk material (8 ⁇ 8 ⁇ 8) in xz plane; (B) deformation pattern for bulk material (8 ⁇ 8 ⁇ 8) in yz plane; (C) deformation pattern for the representative volume unit (2 ⁇ 2 ⁇ 2) in xz plane; and (D) an isometric view of the deformation pattern for the representative volume unit (2 ⁇ 2 ⁇ 2).
  • FIG. 9 provides the geometric configurations for a three dimensional structure porous metamaterial with NLC according to the second embodiment of the present invention showing (A) the optimised building cell from BESO; (B) the simplified building cell unit; and (C) a block of the comparative material comprising an 8 ⁇ 8 ⁇ 8 matrix of the building cell unit.
  • FIG. 10 provides a comparison of deformation pattern of inventive NC metamaterial with NLC shown in FIG. 9 between (A) experiment and (B) finite element model; and (C) Comparison of strain-pressure history between FE results and experimental data for NLC material under pressure.
  • FIG. 11 provides the geometric configurations for a three dimensional structure porous metamaterial with NAC according to the second embodiment of the present invention showing (A) the optimised half building cell from BESO; (B) the optimised building cell from BESO; (C) the simplified building cell unit; and (D) a block of the material comprising an 8 ⁇ 8 ⁇ 8 matrix of the building cell unit.
  • FIG. 12 provides the geometric configurations for a three dimensional structure porous metamaterial with ZLC according to the second embodiment of the present invention showing (A) the optimised half building cell from BESO; (B) the optimised building cell from BESO; (C) the simplified building cell unit; and (D) a block of the material comprising an 8 ⁇ 8 ⁇ 8 matrix of the building cell unit.
  • FIG. 13 provides the geometric configurations for a three dimensional structure porous metamaterial with ZAC according to the second embodiment of the present invention showing (A) the optimised half building cell from BESO; (B) the optimised building cell from BESO; (C) the simplified building cell unit; and (D) a block of the material comprising an 8 ⁇ 8 ⁇ 8 matrix of the building cell unit.
  • the present invention generally relates to a series of 3D structured porous metamaterial with specific deformation pattern under applied loading, and more particularly a 3D structured porous metamaterial having at least one of:
  • the initial design of the microstructure of an auxetic metamaterial form of the present invention originates from using a three-dimensional repeating matrix formed from a base unit comprising a platonic solid such as a cube having a shaped void space such as a sphere or ellipsoid.
  • the platonic solid provides a repeatable and stackable base structure, and the shaped void imparts the required characteristic to the void space and the surrounding base unit framework structure (around the void).
  • the void geometry of each base unit is tailored to provide a porosity of between 0.3 and 0.97; and provide the metamaterial with a response under tension and compression having a Poisson's ratio of 0 to ⁇ 0.5. The specific porosity depends on the type of shaped void used.
  • the porosity is typically between 0.69 and 0.97 for a spherical shaped void; between 0.30 and 0.90 for regular non-spherical shaped voids; or between 0.3 and 0.97 for optimised shaped voids.
  • this structure imparts a tailored deformation character to the material, with the negative Poisson's ratios achieved through the a specific deformation characteristic of the voids (alternating opening and closing pattern of adjacent voids) in the material combined with the spatial rotation and translation of a rigid part of base unit material accompanied by the bending and stretching of the thinner or more flexible part of the base unit material.
  • the initial design of the microstructure of the zero or negative compressibility (NC) metamaterial form of the present invention originates from using a three-dimensional repeating matrix formed from a base unit comprising a platonic solid, such as a cube, having one or more shaped void spaces.
  • the shape of the voids within that base unit and thus the topology of those building unit is derived from a bi-directional evolutionary structural optimization (BESO) model formed to provide the desired NC properties using the desired base unit (again for example a cube). That BESO result is then altered to simplify the topology of the void or voids to have a more regular shape. This simplified shape is typically more suitable for 3D printing construction.
  • BESO evolutionary structural optimization
  • the platonic solid provides a repeatable and stackable base structure, and the shaped void or voids in the base unit cell (an optimised shaped void) imparts the required characteristic to the void space and the surrounding base unit framework structure (around the void).
  • the void geometry (the optimised shape of the void or voids) of each base unit is tailored to provide a porosity of between 0.3 and 0.95; and provide the NC metamaterial with a response under uniform pressure having one of the following behaviour: NLC, NAC, ZLC and ZAC.
  • the material of the base unit can be polymeric including, but not limited to, unfilled or filled vulcanized rubber, natural or synthetic rubber, cross-linked elastomer, thermoplastic vulcanizate, thermoplastic elastomer, block copolymer, segmented copolymer, cross-linked polymer, thermoplastic polymer, filled or unfilled polymer, or epoxy.
  • the material of the base unit but may also be non-polymeric including, but not limited to, metallic and ceramic and composite materials. Exemplary metals include aluminium, magnesium, titanium, iron and alloys thereof.
  • Fabrication of 3D structures according to the present invention can be achieved through 3D printing, dissolving or melting patterned voids from a base material and sintering techniques well known in the art.
  • the optimization method used for the initial design of the microstructure of the zero or negative compressibility (NC) metamaterial form of the present invention is based on the bi-directional evolutionary structural optimization (BESO).
  • BESO bi-directional evolutionary structural optimization
  • the ground structure is a unit cubic cell and the material properties (e.g. elasticity matrix) is determined using the homogenization theory.
  • the BESO method was applied to the design of materials of four types, namely, NLC, NAC, zero linear compressibility (ZLC) and zero area compressibility (ZAC).
  • a cellular material consisting of a base material and voids is often modelled as a microstructure of a periodic base cell (PBC) using finite element (FE) analysis.
  • PBC periodic base cell
  • FE finite element
  • E is the elastic matrix of the base material
  • NE is the number of elements
  • ⁇ i 0 is the i-th unit strain field
  • ⁇ i is the corresponding induced strain field
  • the homogenized compliance matrix C H is the inverse of E H , i.e.
  • the area compressibility in the ij plane is defined as
  • ⁇ Aij ⁇ Li + ⁇ Lj , i ⁇ j (5)
  • Eq. (6) is the summation of the nine constants of the compliance matrix in Eq. (3), which is numerically equivalent to twice the strain energy of the microstructure under the unit hydrostatic stress. Since the strain energy is greater than or equal to zero, it is clear that for orthotropic materials the volume compressibility can either be positive or zero.
  • a typical optimization problem is usually defined in terms of the objective function(s) and constraints(s).
  • an obvious choice of the objective function is the linear compressibility in a particular direction.
  • ⁇ L3 C 31 +C 32 +C 33 .
  • ⁇ L3 is initially positive and one way to “drive” it to become negative is to increase the weighing of the two negative terms, i.e.
  • ⁇ L3 ⁇ (p
  • the lower bound of p is 1, which must be reached on convergence.
  • the upper bound of p is specified by assuming the linear compressibility equal to zero, i.e.
  • the value of p is to be determined. Because of the same p value being applied to axes 1 and 2, the resulting material is to be symmetrical to the 45 degree line in plane 1-2.
  • the stiffness in axis 3 is maintained by including C 33 in the objective function.
  • the stiffness in axes 1 and 2 can be considered by specifying a constraint on C 11 and C 22 , for example, by requiring them to be less than 1/E*, where E* is a prescribed stiffness target.
  • V is the prescribed volume
  • V e is the volume of element e
  • the sensitivity of elasticity constants can be obtained by using the adjoint method (Bendsee, M. P., Sigmund, O., 2003. Topology optimization: theory, methods and applications 2nd ed. Springer, Berlin). From Eq. (1), the sensitivity of E ij H can be expressed as
  • E ⁇ ( x e ) E b ⁇ ⁇ 1 + x e ⁇ ( E b ⁇ ⁇ 2 - E b ⁇ ⁇ 1 ) 1 + q ⁇ ( 1 - x e ) ( 12 )
  • the present study is focused on designing cellular materials and therefore one of the base materials is void, i.e. either E b1 or E b2 is approaching zero.
  • the sensitivity of the mean compliance matrix C H is calculated by using the chain rule, i.e.
  • the above sensitivity analysis forms the basis of the sensitivity number which is used as the search criterion in the BESO solution process. From Eq. (10), the sensitivity number is defined as
  • the sensitivity number ⁇ e is then filtered through a spherical range of radius r min to obtain a weighted ‘average’, i.e.
  • the sensitivity of the compliance matrix is filtered in the same way, i.e.
  • BESO performs the search for the optimal solution iteratively until certain criteria are satisfied. Details of the solution procedure are as follows:
  • the modification according to the threshold is conducted as follows. First, sort the sensitivity numbers of the NE elements in a descend order. Then void elements above the threshold NE thre are switched to solid, and solid elements below the threshold are switched to void. As a result, the total numbers of elements removed and added are NR and NA, respectively.
  • the net number of modified element is NR ⁇ NA, which is positive if the volume is approaching from the initial high value to the target.
  • NR ′ R max ⁇ NE ⁇ NR NR + NA ( 21 )
  • NA ′ R max ⁇ NE ⁇ NA NR + NA ( 22 )
  • the outer loop of the BESO procedure is as follows:
  • the Lagrangian function ⁇ L has two unknowns, namely the stress factor p and the Lagrangian multiplier ⁇ associated with the stiffness constraints. If the constraint is too stringent, i.e. the value of
  • the stress factor p is solved by a general bi-section method:
  • ⁇ L2 1 ⁇ 2( pC 21 +C 23 +pC 12 +C 32 +2 C 22 ) (27b)
  • the overall BESO procedure (outer-loop) is similar to that described previously. At each iteration an inner loop is conducted to solve the Lagrangian multiplier ⁇ . Its value is then averaged between the current and last iterations.
  • the procedure to determine A is as follows.
  • This function is used to calculate the sensitivity of the subsequent iteration in the outer-loop.
  • This function is used to calculate the sensitivity of the subsequent iteration in the outer-loop.
  • the geometry of the base cell for this example 3D auxetic metamaterial is formed by creating a hollow spherical cavity inside a cube, as shown in FIG. 1A (A) and FIG. 1B (A). Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 1A (B) and FIG. 1B (B).
  • the experimental bulk metamaterial was constructed by repeating nine building cells along three normal directions and cut half of the both end-cells in each direction.
  • Each of the specimens of the bulk 3D material were manufactured using 3D printing (ObjetConnex350) with a silicone-based rubber material (TangoPlus) and a supporting material.
  • the Representative Volume Element contains four building cells as shown in FIG. 1A (C) and FIG. 1B (C). According to the ratio (R) of the diameter of the sphere to the length of the cube, two resultant geometry were established:
  • a comparison of the deformation patterns between the experimental (A) and model (B) is provided in FIG. 4 . Comparative Force-Displacement curves of the experimental (A) and model (B) are shown in FIGS. 5A and 5B .
  • the performance of the inventive 3D cubic metamaterial was tested using standard compression tests similar to those commonly used for other cellular materials.
  • FIG. 2 Two samples of the inventive cubic cell material are shown in FIG. 2 .
  • the left sample (A) still includes supporting material for the 3D printing.
  • the right sample (B) has the supporting material removed. In spite of extreme care being taken during the removal of the supporting material, a few of the thinnest links in the bulk material were broken. An epoxy adhesive was used to repair that damage.
  • Comparative compression tests between the (1) comparative face-centred cubic cell and (2) the inventive cubic cell were conducted at a fixed strain rate of 10 ⁇ 3 s ⁇ 1 using a Shimazu machine. Two cameras were used to capture the deformation in two lateral directions so as to determine the evolution of the Poisson's ratio of the metamaterial.
  • the end strain were fixed at a nominal strain up to 0.3 for specimen formed from the comparative face-centred cubic building cells and 0.5 for specimens with inventive cubic building cells to avoid potential damage of the specimens. It was found that within these strain ranges, the deformation was purely elastic and totally reversible.
  • the bulk material composed of the comparative face-centred cubic building cells only exhibited global buckling at a very large strain of 0.25 as shown in FIG. 3A (a). Furthermore, as shown in FIG. 3B the stress-strain curve is linear before the buckling occurs. No obvious auxetic behaviour was observed.
  • the Applicant notes that the localised buckling modes with alternating ellipsoids similar to 2D NPR materials reported in for example Overvelde et al (Compaction Through Buckling in 2D Periodic, Soft and Porous Structures: Effect of Pore Shape. Advanced Materials. 2012; 24:2337-2342) did not occur for this type of material.
  • the bulk material composed of the inventive cubic building cells showed localised buckling modes with alternating ellipsoids. This material therefore deformed with clearly observable auxetic behaviour as shown in FIG. 3A (b). Furthermore, the force-deflection response of inventive metamaterial (shown in FIG. 3C ) in two different directions D 1 and D 2 also shows observable auxetic response.
  • the different buckling behaviour of the materials formed from the face-centred cubic building cell and inventive cubic cell indicates that there is a critical porosity or volume fraction for the desired buckling mode.
  • auxetic behaviour is not possible when the porosity of the material is below 0.60, for example the face-centred cubic building cell material.
  • the Applicants have unexpectantly found that a porosity of at least 0.6, preferably between 0.6 and 0.9 is necessary for the 3D material to display auxetic behaviour.
  • the ABAQUS/standard solver was employed for buckling analysis and ABAQUS/explicit solver was employed for postbuckling analyses.
  • Quadratic solid elements with secondary accuracy (element type C3D10R with a mesh sweeping seed size of 0.4 mm) were used.
  • the analyses were performed under uniaxial compression.
  • the buckling mode with 3D alternating ellipsoidal pattern from buckling analysis was used as the shape change or imperfection factor for non-linear (large deformation) post-buckling analysis.
  • the finite element models were validated using experimental results.
  • FIG. 4 shows the comparison of deformation process of the metamaterial from numerical simulation and experimental result from one direction.
  • Both the experimental results (A) and modelled behaviour (B) exhibit the auxetic behaviour in a similar manner.
  • a noticeable difference is the long axis of ellipsoid of the representative volume unit (marked with dots) in the centre of the specimen.
  • the similarity remained in the other lateral direction.
  • these two different deformation patterns have nearly identical eigenvalues. Based on this analysis, the inventors consider that the actual deformation pattern after buckling is determined by the imperfection of the initial geometry.
  • the buckling mode was influenced by the boundary conditions of the FE model. Two boundary conditions were examined. One constrains all freedoms of the nodes on top and bottom surface except for the freedom on loading direction on the top surface and the other constrains only the freedom of the nodes bottom surface along loading direction.
  • the first buckling mode from the numerical simulation exhibited local buckling with alternating ellipsoids.
  • the first buckling mode exhibited a planar pattern which was similar to the deformation patterns observed previously by Willshaw and Mullin (Soft Matter. 2012, 8, 1747). The 3D buckling pattern occurred as the fifth buckling mode.
  • the geometry of the base cell for this example 3D auxetic metamaterial is formed by creating a hollow ovoid cavity inside a cube, as shown in FIG. 6 .
  • the designed ovoid comprised an 8% imperfection in the shape of the spherical void used in the material discussed in Examples 1 and 2.
  • the matrix of base units in the material was arranged such that the central length axis of the ovoid void of each base unit was perpendicular to the central length axis of the ovoid void of each adjoining base unit. This, in effect, introduced the pattern of the buckling mode seen in Examples 1 and 2 into the void pattern of this embodiment of the metamaterial.
  • the porosity of this unit cell was found to be 87.4% for Example 1 and 87.2% for Example 2.
  • FIG. 5 A direct comparison of nominal stress-strain curves between experimental and numerical results is shown in FIG. 5 . Both curves exhibit a similar trend and the corresponding stress are at similar levels. This demonstrates general agreement between experimental results and the finite element model. It is noted that the lower stress level in the experimental results could be attributed to the broken links during the removal process of the supporting material.
  • the stress-strain curves of the inventive metamaterial are similar to the other cellular materials undergoing plastic deformation, with the difference that deformation of the inventive metamaterials is purely elastic and fully reversible. This appears to be an attribute of the properties of the base material used.
  • the Applicant observes that if the magnitude of the imperfection in the spherical shape of the void is increased (and thus the shape of the ovoid void altered or flattened), the Poisson's ratio of material could be altered, and thus effectively tailored to a desired value. This would produce a series of inventive cubic 3D metamaterials with prescribed initial negative Poisson's ratio value. This approach provides a fundamentally new way for generating a serial of 3D materials with a desired initial value of negative Poisson's ratio.
  • volume fraction for the base cell and representative volume element of the inventive metamaterial varies with different imperfection magnitude.
  • a combination of this approach with the initial geometry design can therefore be considered to design metamaterials with a desired volume fraction.
  • the configuration of the base unit, void geometry and pattern of the matrix formed from the base units can be tailored using a buckling mode obtained through finite element analysis.
  • the introduction of the buckling pattern into the matrix of the material and varying the magnitude of the imperfection in the spherical shape of the void enables so that it provides a mean to tailor the initial value of Poisson's ratio in a range from 0 to ⁇ 0.5.
  • a metamaterial of the present invention can also be formed using a cubic base cell with other void shapes, such as tetrahedron, or ellipsoid.
  • FIG. 7A provides the geometric configurations for a three dimensional structure porous metamaterial with tetrahedron in cube building cell.
  • FIG. 7B provides the geometric configurations for a three dimensional structure porous metamaterial with ellipsoid in cube building cell.
  • the geometry of the base cell for this example 3D auxetic metamaterial is formed by creating a hollow tetrahedron or ellipsoid cavity inside a cube, as shown in FIG. 7A (A) and FIG. 7B (A).
  • Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 7A (B) and FIG. 7B (B).
  • FIG. 7A (C) and FIG. 7B (C) illustrate a representative volume unit of the inventive metamaterial.
  • the porosity of this type of unit cell was found to be 0.63 in FIG. 7A (in the range of 0.5 to 0.91) and 0.69 in FIG. 7B (in the range of 0.6 to 0.97).
  • FIG. 8A provides the deformation pattern of the metamaterial shown in FIG. 7A under load.
  • FIG. 8B provides the deformation pattern of the metamaterial shown in FIG. 7B under load.
  • the deformation pattern shown in FIGS. 8A and 8B illustrates similar behaviour with previous cubic base cell with spherical voids examples.
  • NC metamaterial of the present invention can be formed using a frame work similar to the topology resulting from bi-directional evolutionary structural optimization (BESO).
  • FIG. 9 provides the geometric configurations for the resulting three dimensional structure porous NC metamaterial with NLC.
  • FIG. 9(A) provides the topology obtained from BESO.
  • the geometry of the building cell for this example 3D NC metamaterial is formed by simplifying the irregular members in FIG. 9(A) to a truss with varied cross-section maximized at the middle span.
  • the simplified building cell is shown in FIG. 9(B) .
  • Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 9(C) .
  • the porosity of this unit cell was found to be 0.902.
  • the basic form of the NLC metamaterial is derived from BESO calculations, as discussed previously in relation to NLC optimisation.
  • the finite element analysis is conducted by using ABAQUS version 10.1. Due to symmetry in three directions for orthotropic materials, only one eighth of the unit cell needs to be modelled. The one eighth model is divided into a mesh of 30 ⁇ 30 ⁇ 30 brick elements (element type: C3D8). The resulting topology is smoothened based on curve and surface fitting. The target volume V is 30%. The unit for the compressibility is Pa ⁇ 1 .
  • E b1 10 ⁇ 15 (void)
  • E b2 1 (solid).
  • the procedure has designed an optimised shaped void comprising a regular but complex shape, providing a cutout aperture in the truss structure, and an open end.
  • a numerical simulation of a stress test was conducted on a model constructed from 8 ⁇ 8 ⁇ 8 unit cells of the above topology.
  • the model is resized to 100 mm ⁇ 100 mm ⁇ 100 mm and was meshed with 7424 quadratic tetrahedral elements (ABAQUS element type C3D10I).
  • X, Y and Z directions correspond to axes 1, 2 and 3, respectively.
  • the material properties of the TangoPlus material are measured through standard compression test on three printed cylindrical samples, with the true strain up to 0.70. The results indicate that the constitutive behaviour of the base material can be accurately represented by a linear elastic model. It is found that the Young's modulus is 1.05 MPa and the Poisson's ratio is 0.48. These values are used in the FE simulations described below.
  • FIG. 11 provides the geometric configurations for a three dimensional structure porous negative compression metamaterial with NAC.
  • FIGS. 11 A—half cell
  • 11 B—full unit cell
  • BESO bi-directional evolutionary structural optimization
  • the porosity of this unit cell was found to be 0.696.
  • the resulting topology is shown in FIGS. 11(A) and 11(B) .
  • the topology is symmetrical with respect to the 45 degree plane perpendicular to plane 2-3.
  • the procedure has designed an optimised shaped void comprising multiple voids within the cubic base unit forming a complex shape.
  • the optimised shape void includes two internal voids and three external voids forming the topology of the base building cell.
  • the geometry of the building cell for this example 3D NC metamaterial is formed by simplifying the irregular members in FIG. 11(B) to a truss with varied cross-section maximized at the middle span.
  • the simplified building cell is shown in FIG. 11(C) .
  • Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 11(D) .
  • FIG. 12 provides the geometric configurations for a three dimensional structure porous negative compressibility metamaterial with ZLC.
  • FIG. 12 (A—half cell) and FIG. 12 (B—full cell) provides the topology obtained from bi-directional evolutionary structural optimization (BESO).
  • BESO bi-directional evolutionary structural optimization
  • Example 5 the procedure provided in Example 5 was used to determine a ZLC design.
  • the linear compressibility ⁇ L3 equals ⁇ 0.002, which is very close to zero.
  • the procedure has designed an optimised shaped void comprising multiple voids within the cubic base unit forming a complex shape.
  • the optimised shape void includes two internal voids and at least three external voids (sides) forming the topology of the base building cell.
  • the geometry of the building cell for this example 3D NC metamaterial is formed by simplifying the irregular members in FIG. 12(B) to a truss with varied cross-section maximized at the middle span.
  • the simplified building cell is shown in FIG. 12(C) .
  • Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 12(D) .
  • the porosity of this unit cell was found to be 0.854.
  • FIG. 13 provides the geometric configurations for a 3D structure porous negative compressibility metamaterial with ZAC.
  • FIGS. 13 (A—half cell) and 13 (B—full unit cell) provides the topology obtained from BESO.
  • the strain energy is 7.00, which is higher than that of ZLC (6.33). This is because of the additional constraint on ⁇ L2 compared to the ZLC design.
  • the area compressibility ⁇ A23 is equal to ⁇ 0.002, which is negligibly small (in terms of its absolute value) compared to that of the NAC design shown in FIG. 11 (Example 6) ( ⁇ 25.40).
  • the procedure has designed an optimised shaped void comprising multiple voids within the cubic base unit forming a complex shape.
  • the optimised shape void includes an internal void and at least two external voids (sides) forming the topology of the base building cell.
  • the porosity of this unit cell was found to be 0.893.
  • the geometry of the building cell for this example 3D NC metamaterial is formed by simplifying the irregular members in FIG. 13(B) to a truss with varied cross-section maximized at the middle span.
  • the simplified building cell is shown in FIG. 13(C) .
  • Each of the building cells was repeated to form a 3D cellular material as respectively shown in FIG. 12(D) .
  • the material of the present invention can be used to fabricate sensors, actuators, prosthetics, surgical implants, anchors, (as for sutures, tendons, ligaments, or muscle), fasteners, seals, corks, filters, sieves, shock absorbers, impact-mitigating materials, hybrids, or structures, impact absorption or cushioning materials, hybrids, or structures, wave propagation control materials, hybrids, or structures, blast-resistant materials, hybrids, or structures, micro-electro-mechanical systems (MEMS) components, and/or stents.
  • MEMS micro-electro-mechanical systems
  • Applications of this invention directed at the biomedical field include uses relating to prosthetic materials, surgical implants, and anchors for sutures and tendons, endoscopy, and stents.
  • Applications of this invention directed the mechanical/electrical field include uses in piezoelectric sensors and actuators, armours, cushioning, and impact and blast resistant materials, as deployable material and defence materials for infrastructures, the filter and sieve field, the fastener field, the sealing and cork fields, and the field of micro-electro-mechanical systems (MEMS).
  • MEMS micro-electro-mechanical systems
  • the inventive metamaterial can be formed as a compressible biocompatible polymer for use in intervertebral disc replacement.
  • the configuration and patterning the voids can be configured to allow the flow of fluid.
  • the fluid can be used as a dampening mechanism within the material.
  • NLC/NAC metamaterials An immediate application of NLC/NAC metamaterials is the optical component in interferometric pressure sensors due to the higher sensitivity achieved by a combination of large volume compressibility with negative linear compressibility.
  • NC metamaterials One significant application of the NC metamaterials is to be used as inserted foam for the OA treatment surgery using a NPWT system.
  • the NC metamaterial will maintain their height but contract laterally under negative pressure and thereby enable the OA wound to close directly without using invasive mechanical devices.
  • NLC/NAC materials also have potential to be used as efficient biological structures, nanofluidic actuators or as compensators for undesirable moisture-induced swelling of concrete/clay-based engineering materials (Cairns et al., 2013).
  • the inventive metamaterials can be used in a new type of smart acre for defence engineering or in blast control from explosive devices and projectiles.
  • the inventive material is formed from a Titanium or titanium alloy base unit matrix. The material can be used to compresses to the point of impact thereby providing lightweight armour plating.
  • the material can be used as lightweight cellular materials with enhanced energy absorption for motor vehicles.

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